This invention relates to a speed control apparatus for an automotive vehicle. More particularly, it relates to a speed control apparatus which is capable of performing cruise control at high vehicle speeds, creep control at low vehicle speeds, and wheel slip suppression at all vehicle speeds.
Cruise control devices which maintain a constant vehicle speed when a vehicle is running at high speeds have existed for many years. Recently, creep control devices for low vehicle speeds have been proposed. A creep control device is a device which maintains the speed of a vehicle constant when the vehicle is "creeping", i.e., running at low speeds, such as when it is travelling in congested traffic. For example, Japanese Published Unexamined Patent Application No. 61-247525 discloses a creep control device in which, during creep control, a control unit adjusts the degree of opening of a throttle valve so as to maintain a constant, set vehicle speed as long as the accelerator pedal is depressed. During creep control, the vehicle speed is independent of the amount of depression of the accelerator pedal. When the driver removes his foot from the accelerator pedal, the control unit automatically applies the brakes and stops the vehicle. That apparatus has the disadvantage that the driver can not control the creep speed (the vehicle speed during creep control), which is set in advance.
Japanese Published Unexamined Patent Application No. 62-255252 discloses a creep control apparatus in which the throttle valve and the brakes of a vehicle are both controlled by a control unit so as to maintain a predetermined creep speed. Creep control is performed only when the driver is not stepping on either the accelerator pedal or the brake pedal. That apparatus has the disadvantage that the creep speed is set in advance and can not be easily adjusted by the driver. It also has the disadvantage that since it controls both the- throttle valve and the brakes of a vehicle, it tends to be large and expensive.